The aim of this research is to investigate certain aspects of the molecular mechanism of gene expression in mammalian and other eucaryotic cells. We propose to continue our studies of the organization of nuclear ribonucleoprotein complexes containing hnRNA (hnRNP) sequences, and the role these structures play in the correct processing and splicing of mRNA sequences. Our previous results suggest a structure for hnRNP in which the large RNA molecule is folded on distinct protein complexes to give a repeating 30S RNP substructure. Most hnRNA sequences are bound to these 30S subcomplexes but certain distinctive RNA regiona are exposed or complexed with proteins other than the relatively simple set of polypeptides found in the 30S particles; for example, selfcomplementary, or double-stranded RNA regions appear to be exposed while nuclear poly(A) is contained in a discrete 15S substructure within hnRNP. Recently, we have begun a study of the distribution of snRNPs in relation to hnRNP to assess their role in RNA processing. We are now investigating the chemistry of the hnRNP proteins and using dissociation-reconstitution systems to analyze the specificity of protein:protein and protein:RNA interactions. Our polyclonal and monoclonal antibodies are used to examine the physiology of hnRNP and snRNP complexes. The distributions of hnRNP and snRNP complexes determined at the light and electron microscope levels differ significantly during the cell cycle and variations in the stoichiometry and metabolic properties of RNP proteins suggest that they may be significant factors in the modulation of growth and differentiation of normal and transformed cells.